Thermal Physics (NEED TO ADD RP) Flashcards
Internal energy
The sum of the randomly distributed kinetic energies and potential energies of the particles in a body
KE + PE
What causes the kinetic energy in the molecules all substances
The speed of the molecules
What causes the potential energy in the molecules of all substances
The separation between the molecules
What does the amount of KE and PE a substance contains depend on
Its phase of matter (solid, liquid or gas)
Why do all particles have different speeds and separations
The particles are randomly distributed
What is the internal energy of a system determined by
Temperature - high temp = high KE
Random motion of molecules
Phase - gases have highest internal energy
Intermolecular interactions between particles
How can the internal energy of a system be increased
Doing work on it
Adding thermal energy to it (heating it)
How can the internal energy of a system be decreased
Losing thermal energy to its surroundings
Changing state from a gas to a liquid or liquid to solid
During a change of state what happens to the different energies of the particles
The potential energies of the particle ensemble are changing but NOT the kinetic energies
First law of thermodynamics
The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it
Which situations does the first law of thermodynamics apply to
ALL situations not only gases
What happens to the internal energy of the gas and why, when a gas expands
Work is done by the gas on the surroundings and this decreases the internal energy of the gas
What happens to the internal energy of a gas and why, when a gas is compressed
Work is done on the gas by the surroundings and this increases the internal energy of the gas
What happens when a piston moves down on the cylinder containing a gas
It compresses the gas, work is done on the gas
The molecules are pushed closer together
Therefore, they have higher kinetic energy as they move faster
This increases the internal energy of the gas
What happens when a piston moves up a cylinder containing a gas
It expands the gas, work is done by the gas
The molecules are spread further apart
Therefore, they have lower kinetic energy as they move slower
This decreases the internal energy of the gas
How does heating the gas increase the internal energy by the same amount as doing work
Increasing the temperature of the gas means the molecules move around faster.
Therefore they have higher kinetic energy and increased internal energy
How does the number of molecules in the container change if the gas is expanding or contracting
IT DOESN’T. Number of molecules in the container ALWAYS stays the same
Specific heat capacity of a substance
The amount of thermal energy required to raise the temperature of 1kg of a substance by 1 degrees C without a change of state
Equation for specific heat capacity
Change in thermal energy = mass of substance being heated x specific heat capacity of substance x change in temp
How does heating and cooling of a substance vary with its specific heat capacity
Low specific heat capacity = Heats and cools down quickly as less energy needed to change its temp
High specific heat capacity = Heats and cools down slowly
What piece of equipment can be used to find the specific heat capacity of a fluid
A continuous-flow calorimeter
How does a continuous flow calorimeter work
A fluid flows continuously over a heating element where energy is transferred to the fluid.
It is assumed the heat transferred from apparatus to surroundings is constant
What variables are changed and kept constant in the continuous flow calorimeter experiment
Flow rate and pd is changed
Change in temperature of the fluid is constant
How to find mass of fluid in a continuous flow calorimeter
Record the flow rate
Multiple flow rate by the time taken to give the mass of the fluid that flows in as m1
How to find the electrical energy supplied to the fluid for the first flow rate in time t
current(1) x voltage(1) x time(1) = Q(1) = m(1) x c x change in temp + energy lost
What is the temperature change when a substance changes state
There is NO temperature change
Latent heat
The thermal energy required to change the state of 1kg of mass of a substance without any change of temperature
2 types of latent heat
Specific latent heat of fusion - melting a solid or freezing a liquid
Specific latent heat of vaporisation - vaporising a liquid or condensing a gas
What do the flat sections on a temp/heat supplied graph represent
The latent heats of ….
Equation for specific latent heat
Amount of thermal energy to change state = mass of substance changing state x latent heat of ….
Why is more energy needed to evaporate 1kg of water than to melt the same amount of ice into water
To melt ice, energy is required to just increase the molecule separation till they can flow freely over each other
To boil water, energy is needed to completely separate the molecules till there are no longer forces of attraction between the molecules.
Why is latent heat of vaporisation of water much greater than specific latent heat of fusion of water
More energy has to be supplied to separate molecules than break a solid bond
What energy changes take place during a change of state
Potential energies of the molecules change but NOT their kinetic energies
What is kinetic energy proportional to
Temperature
Absolute zero
The temperature at which the molecules in a substance have zero kinetic energy. It is the lowest temperature possible.
-273 degrees celscius
How to convert celsius to Kelvin
Celsius = Kelvin - 273
In the specific heat capacity equation, why is no unit needed for temperature
the difference in temperature between the 2 values will be exactly the same
What is assumed in Boyles, Charles’ and Pressure Law
The mass and number of molecules of the gas are assumed to be constant for each of these laws
Boyle’s Law
Pressure is inversely proportional to the volume of a gas
OR:
P1V1 = P2V2
ONLY IF TEMPERATURE OF IDEAL GAS IS CONSTANT
Condition needed for Boyle’s Law
Temperature of the ideal gas must be constant
Units in Boyle’s Law equation
Pressure - Pa
Volume - m^3
What does a graph of Boyle’s Law of volume against pressure look like
Curve downwards, not touching either axis
What would a Boyle’s Law graph look like if temperature is higher but still constant
The graph has the same shape but is shifted above the origin/ is higher up
Charle’s Law
Volume is directly proportional to the temperature of a gas (V = kT)
OR:
V1/T1 = V2/T2
ONLY IF PRESSURE OF GAS IS CONSTANT
Condition needed for Charles’ Law
Pressure of the ideal gas must be constant
Units for temperature in Charles’ Law
Kelvin
What does a graph of Charles’ Law look like for volume against temperature
Straight line through the origin. This shows direct proportionality
What is an ideal gas
Obeys ideal gas law at all temperatures
Collisions of molecules are elastic
Molecules have negligible volume
No interactions between molecules
How do molecules in a gas exert a pressure on a surface
Molecules move around randomly at high speeds, colliding with surfaces and exerting pressure on them.
Describe what happens when the temperature of a gas is increased, to the pressure
The molecules move faster and so collide with the surface of the walls more frequently. Each collision applied a force across the SA of the walls. The faster the molecules hit the walls, the greater the force on the,. Therefore pressure increases
What happens to the pressure when the volume of a box decreases and temperature stays constant and why
There will be a smaller surface area of the walls and hence more collisions. Therefore, pressure increases
Pressure in an ideal gas
The force of collisions of the gas molecules per unit area of a container
Ideal Gas equation (1)
pV = nRT
Units in the ideal gas equation
Pressure = Pa
Volume = m^3
number of moles = moles
Temperature = Kelvin
Ideal Gas equation (2)
pV = NkT
where N is the number of molecules and k is the Boltzmann constant
What happens to the walls of a container when a gas expands
It does work on its surroundings by exerting pressure on the walls
Work done when a volume of gas changes at constant pressure =
Pressure of gas x (increase in) Volume of gas
Number of gas particles =
number of moles x Avogadros constant
number of moles =
Mass of gas sample / molar mass
mass =
molar mass / avogadros constant = mass of sample / number of gas particles
Boltzmann constant =
Molar gas constant / Avogadros number
Why is the value for Boltzmann constant very small
The increase in KE of a molecule is very small for every incremental increase in temperature
What energies exist in ideal gas molecules
Only KE not potential energy as ideal gas molecules are assumed to have no intermolecular forces
Change in internal energy of an ideal gas =
3/2 k x change in T
Relationship between change in internal energy and change in temperature for ideal gas
Change in internal energy is proportional to change in temperature
Assumptions in kinetic theory
Molecules of a gas behave as identical or have same mass
Molecules of gas are hard, perfectly elastic spheres
Volume of molecules is negligible compared to volume of container
Time of a collision is negligible compared to the time between collisions
There are no intermolecular forces between molecules except during impact
External forces such as gravity are ignored
Molecules move in continuous random motion
Newtons laws apply
There are a very large number of molecules
Elastic collision
Why do we use average speed for molecules
Number of molecules of gas in a container is very large
Elastic collision
No kinetic energy lost in collision
Difference between gas laws and kinetic theory
gas laws are empirical in nature whereas the kinetic theory model arises from theory.
Crms =
square root ( C1^2 + C2^2 + C3^2 C4^2 + …….) / N)
Pressure in terms of Crms =
1/3 x m/V x N(Crms)^2
When does Brownian motion of particles occur
When small particles suspended in a liquid or gas are observed to move around in a random, erratic fashion
What does Brownian motion provide evidence for
The existence of atoms in a gas or liquids
How to see Brownian motion
Under a microscope
What does the random motion of particles (in Brownian motion) mean
A range of speeds
No preferred direction of movement
How to find p2 using temp and intial pressure
P2 = T2 x P1 / T1
P1/T1 = P2/T2
P1 x V1 at constant temp =
P2 x V2
